I downloaded a file whose extension is .INDD. How do I open it?

.INDD is an extension that is used for those types of files that are created by Adobe’s InDesign page layout. In order to open this file you must have "Adobe InCopy professional writing software package". These are used to create books, magazines, newspapers, catalogs, flyers, brochures and any other document where page layout is required.
For more: http://www.softango.com/extensions/indd